PROJECT SUMMARY/ABSTRACT Nonalcoholic fatty liver disease (NAFLD) and the related disorder nonalcoholic steatohepatitis (NASH) are now the most prevalent liver diseases in the world. Research into the pathophysiology of NAFLD/NASH has uncovered mechanisms by which lipids can accumulate in liver cells and cause liver injury ? but has also revealed the tremendous complexity of the disease and its variability among global populations. Mouse models of NAFLD/NASH, despite their many benefits, fall short of replicating the human disease. For this reason there is an important need to study the pathogenesis of NAFLD/NASH directly in humans. The goal of this exploratory/developmental research project is to use human induced pluripotent stem cells (iPSC) to promote better understanding of NAFLD/NASH. Our proposed strategy is to take human iPSC, generated from 48 patients with biopsy-proven NAFLD/NASH and 19 healthy controls, and study them in culture after differentiating them to iPSC-derived hepatocytes (iPSC-heps). At the core of the research plan is the hypothesis that certain critical features of human NAFLD/NASH are hepatocyte-autonomous. In other words, we believe iPSC-heps from NAFLD/NASH patients will exhibit phenotypic differences from control iPSC-heps that underlie their predisposition to injury and disease. We will test this hypothesis by completing several Specific Aims. In Aim 1 we will determine whether iPSC-heps from NAFLD/NASH patients are distinguishable from iPSC-heps from healthy subjects at baseline, without any exogenous stimulus. This will reveal whether liver cells from NAFLD/NASH patients exhibit an inherent predisposition to disease that manifests without provocation. In Aim 2 we will assess whether iPSC-heps from NAFLD/NASH patients are more sensitive to metabolic stresses than iPSC-heps from healthy subjects. Together, Aims 1 and 2 will establish whether iPSC-heps hold promise as a means for expanding knowledge about a complex human liver disease. While conducting studies on the existing collection of iPSC we will pursue a 3rd Aim to expand our cohort, by collecting clinical information and blood from additional NAFLD/NASH patients and healthy controls. We will store PBMC from these individuals with the longer term goal of broadening our iPSC research to larger numbers of subjects. This research project promises to provide valuable information about NAFLD/NASH pathogenesis with direct relevance to NAFLD/NASH patients. Importantly, success with these studies will also prove that research based on iPSC-heps can extend beyond the investigation of individuals with monogenic liver diseases to populations of patients with a complex disease. Positive results will set the stage for more in- depth study of human NAFLD/NASH in family kindreds and/or other populations based on genotype, gender, ethnicity and geography. Overall, the proposed work has the potential to answer important basic research questions about NAFLD/NASH by focusing directly in human subjects through the power of iPSC.